1. Field of the Invention
The present invention relates to binding agents, or binders, for paints, notably exterior paints, or pliable semi-thick coatings particularly well suited for the protective coating of building facades and containing acrylic or styrene/acrylate latices that comprise a benzophenone copolymerizate. By the term "pliable semi-thick coating" is intended a coating material having a consistency similar to that of putty.
2. Description of the Prior Art
Interior paints have long been known to this art that are easy to apply, have minimal odor, and are easy to clean, in which the organic polymer constituting the latex binding agent is "hard," namely, it has a glass transition temperature (Tg) of more than 0.degree. C. These paints applied to exterior surfaces do not collect dust and provide a satisfactory non-sticky surface. However, they exhibit mediocre adhesiveness to the base substrate and have poor resistance to cracking.
Acrylic paints of the type described above are applied to exterior surfaces of buildings, in order to form an effective coating which protects against rainwater penetration, by virtue of their capacity to cover over cracks or micro-cracks in the base, both those that exist currently and those that ultimately will form. The elastomer coating, thus, must necessarily exhibit strong capacities of deformation without the appearance of tears (resistance to cracking) or separation (adhesiveness) of the coating, over a wide temperature range. In addition to its role in protecting against inclement weather, the coating is important from an aesthetic standpoint, which is assessed based on its resistance to soiling principally resulting from the capture and incrustation of dust on the surface of a coating that is excessively soft and to excessively-high surface adhesion. The resistance of the coating to soiling represents, therefore, another very important criterion to take into account in order to evaluate the quality of exterior acrylic paints.
Acrylic paints have indeed been applied to exterior face surfaces, in which the organic polymer composing the latex binding agent is of the "soft" type (Tg of less than 0.degree. C., and preferably lower than -10.degree. C). Consequently, this paint provides proper adhesiveness and resistance to cracking; however, the resistance to dirt and the surface-adhesion property are not satisfactory. One solution to this technical problem is described in U.S. Pat. No. 3,320,198, which relates to an exterior paint whose base is an aqueous acrylic polymer latex containing benzophenone in the proportion of from 0.3% to 10%, and preferably 0.5% to 3% by weight of the weight of the polymer. Nonetheless, to be truly effective, such paints must contain large quantities of benzophenone.
Moreover, benzophenone or derivatives thereof are often solid at ambient temperature, a fact which requires that they be added to the latex as a solution in an organic solvent such as toluene and xylene, or as an aqueous emulsion.
All of these organic compounds (benzophenone and solvents) have a significant vapor pressure, which imparts to the latex a detectable odor which may adversely affect the individual applying the paint. In addition, current ecological guidelines discourage the use of organic solvents.
EP-A-010,000 describes paints similar to those characterized in U.S. Pat. No. 3,320,198. However, to improve the effectiveness of the benzophenone and, therefore, to reduce the quantities used, EP-A-101,000 proposes use of a polymer substituted by radicals that are hardenable by oxidation in air, i.e., alkenyl radicals borne by amide or ester groups.
The paints thus produced have anti-soiling and adhesive properties which remain insufficient, and the benzophenone must still be added as a solution in an organic solvent.